Really produce only a warning for setters with boxed primitive types …

…for primitive properties.

We'd attempted this in cl/659352853, but we ended up reporting a warning and then failing with an error on a later step instead.

With the new test, we see an error until we make the prod change that I've included:

```
Compilation produced the following diagnostics:
foo/bar/Baz.java:11: warning: [AutoValueUnnecessaryBoxing] property method foo.bar.Baz.blam() is primitive but parameter of setter method is not
    Builder blam(Integer x);
            ^
foo/bar/Baz.java:10: error: [AutoValueBuilderMissingMethod] Expected a method with this signature: foo.bar.Baz.Builder blam(int), or a blamBuilder() method
  public interface Builder {
         ^
```

RELNOTES=n/a
PiperOrigin-RevId: 670953681

value/src/main/java/com/google/auto/value/processor/BuilderMethodClassifier.java

@@ -493,7 +493,6 @@ && originalPropertyType(propertyElement).getKind().isPrimitive()) {
             "[%sUnnecessaryBoxing] %s is primitive but parameter of setter method is not",
             autoWhat(),
             propertyString(propertyElement));
-        return Optional.empty();
       }
       return Optional.of(Copier.IDENTITY);
     }

value/src/test/java/com/google/auto/value/processor/AutoValueCompilationTest.java

@@ -4031,6 +4031,7 @@ public void autoValueBuilderNullableSetterPrimitiveGetter() {
         javac()
             .withProcessors(new AutoValueProcessor(), new AutoValueBuilderProcessor())
             .compile(javaFileObject);
+    assertThat(compilation).succeeded();
     assertThat(compilation)
         .hadWarningContaining(
             "property method foo.bar.Baz.blam() is primitive but parameter of setter method is not")